This invention relates generally to frequency synthesizers. Specifically, the invention is the addition, to a known phase-locked loop (PLL) synthesizer, of a subsidiary frequency compression loop (FCL).
Frequency synthesizers are utilized in both transmitters and receivers to provide transmitter oscillator and local oscillator signals, respectively. They generally include a phase-locked loop, having one or more programmable dividers, which locks onto selected harmonics of a reference frequency that is typically crystal controlled. A known frequency synthesizer using a phase-locked loop is shown in FIG. 1.
A signal controlled oscillator such as voltage controlled oscillator (VCO) 20 includes a first output 22 providing the synthesized signal and a second output 24. A mixer 26 mixes the signal from output 24 of VCO 20 with that of a frequency source 28 to provide a down-conversion of the VCO signal on an output line 30 of the mixer. Output line 30 is coupled to the signal input of a programmable divider 32, the divide ratio of which is controlled by frequency controller 34. The output of programmable divider 32 is coupled to a loop phase detector 36 which compares the signal from programmable divider 32 with that of a reference frequency source 38. Phase detector 36 produces a signal on an output line 40 that is a function of the phase difference between the signals coupled thereto from programmable divider 32 and reference frequency source 38. The output 40 of phase detector 36 is coupled through an amplifier and low pass filter 42 to the control input 44 of VCO 20.
In essence, the loop gain of a phase-locked loop is a function of the phase error that is required to get the VCO onto the desired frequency. Thus, if the loop has a high loop gain, a small phase change at the output of the loop's phase detector causes a large swing in the frequency of the VCO signal. The bandwidth of a phase-locked loop is a measure of the loop's response to a phase change of the reference signal. The loop bandwidth is generally determined by the loop's low pass filter, phase detector, and the control sensitivity of the VCO.
As a practical matter, the loop must have a limited bandwidth because an averaged or filtered control signal must be coupled to the control input of the VCO. Based upon various design considerations, such as the desired tracking range of the loop and the modulation to be applied, for a typical synthesizer used in radio equipment selectively operable on radio channels that are spaced at 25 kHz intervals, the bandwidth of the loop cannot be made much wider than 1 kHz.
For synthesizers used in FM communication equipment, it is particularly important to minimize the phase noise, i.e. phase perturbation, of the VCO output signal. Phase noise outside the tracking bandwidth of the loop is mainly determined by the noise of the VCO itself, including contributions to external mechanical disturbances, such as for example, microphonics.
One approach to solving the phase noise problem is to build a very low noise VCO. However, this approach may not be practical due to cost and high power consumption.